Conventionally, glow discharge is utilized as exciting source in analyzing components in samples. When glow discharge is effected between an anode and a sample set on the cathode side, cathode sputtering occurs so that the surface of the sample is dispersed in the discharge region where it is excited for emission. The light thus emitted is taken out for spectrochemical analysis to analyze the components. The glow discharge tube used in the above conventional emission spectrochemical analysis is well known through the disclosure in Japanese Patent Registration No. 760451 (Patent Publication No. SHO49-21680).
The conventional glow discharge tube is schematically shown in FIG. 3. A hollow anode body "A" has a window "W" at the front through which light is taken out. An anode tube "T" extends from the anode body "A" into the cavity in a cathode body "K", and a sample "S" is set so that it closes the opening of the cavity in the cathode body "K". An insulating plate "I" is inserted between the cathode body "K" and the anode body "A" for electrical insulation. Discharge gas such as argon is introduced from an intake port "In" and exhausted from and exhaust port 01 formed in the anode body and an exhaust port 02 formed in the cathode body "K". Thus the glow discharge region is formed in the space in the anode tube "T". The sample constituents dispersed by cathode sputtering and suspended in form of vapor in the discharge region is excited for emission.
Thus, in the conventional glow discharge tube with the anode tube "T" inserted in the cavity in the cathode body "K" and with a gap G4 left between the anode tube "T" and the cathode body "K", the sample is set in such a manner as to close one open end of the cavity so that a glow discharge region is formed between the anode tube "T" and the sample whose potential is maintained at the same level as that of the cathode body "K". With the glow discharge tube of such a construction, however, the discharge gap G3 between the sample and the anode tube end must be set at the specified width while maintaining an appropriate vacuum (2.about.20 Torr) in the discharge region, in order to secure proper glow discharge. To this purpose, discharge gas is forcedly exhausted through the port formed in the circumferential wall of the anode tube during glow discharge in said discharge region.
The sample vapor suspended in the discharge region flows with the exhaust gas stream and sticks to the surrounding walls of the gaps G3 and G4. As a result, the gaps G3 and G4 tend to be bridged, shortcircuiting the anode tube "T" with the sample "S" as well as with the cathode body "K". To prevent such a trouble, the user is often required to dismount the sample "S" and the cathode body "K" from the anode body "A" to clean the surrounding walls of the gaps G3 and G4. If the sample is a zinc-plated steel sheet which produces a large quantity of sputtering, cleaning is required after almost every analysis. In reassembling the discharge tube after cleaning, great care must be taken so that the anode tube "T" does not touch the cathode body "K". Though the analysis itself takes 1.about.3 minutes, the entire operation for analysis requires more than five times as long as that, because the cleaning operation takes about 15 minutes. This is a serious problem when continuous analysis is needed as in the quality control in the production of plated steel sheets. In addition, reproducibility of the analysis with the conventional glow discharge tube is impaired, because the dimensions of the gaps G3 and G4 can change every time the glow discharge tube is disassembled and reassembled. To ensure proper glow discharge without shortcircuiting, the gap G3 between the sample "S" and the anode tube end should be desirably 0.3.about.0.5 mm, which may be somewhat varied according to the sample material, applied voltage and purpose of the analysis. In view of the suitable pressure required for the discharge region (2.about.20 Torr of vacuum is required.), however, it is desirable to make the gap G3 as narrow as possible to effect larger resistance against exhaust gas flow.
In the conventional discharge tube, the dimension of the gap G3 is limited by the discharge gap requirement. The exhaust passage G4 surrounding the anode tube is, therefore, made long and narrow, to increase the exhaust resistance for the discharge region, thereby maintaining the required vacuum. This is why a shortcircuit is caused by the sticking sample vapor as described earlier.